I'd suggest a simple bowl and betta on the front with the question "Happy betta?"

Then on the back a far more dramatic drawing of the betta punching/bursting through the fish bowl glass and the exclamation 'NO!'

A little cartoonie, but dynamic and it'll make people think about it.

I think this is a great idea btw - and I think for teh sites...well, the one that sells teh shirt (if not your own site) might want to be on it. Or if you sell it through art sites (say Deviant Art). Not sure how that all works out tho.

I think on the front of the shirt have a betta in a tiny bowl then on the back have the following

I am not a fish...
I am a living breathing creature who has a personality
I am not a decoration...
I am an animal like you
I am not a pet...
I am a family member
I am a betta and am abused my most owners
Let me show you my true beauty
Give me a loveing home
Do your research so you can let me shine like I am suppose too
Flourish like I was created too
And be loved like I always wanted too
Because I am a betta

I like ill be the judge's idea. You can make the human appear cold and the betta appear cozy and comfortable. It's funny but sends a powerful message. You could also add a thermometer in the bowl stating the temperature. Maybe a calendar in the back ground with really spaced out feeding days and the betta feeding the human leftovers of some sort like a bone with barely any meat on it. Just ideas I think will add to the power